      /* 控制下雪 */
      function snowFall(snow) {
        /* 可配置属性 */
        snow = snow || {};
        this.maxFlake = snow.maxFlake || 200;   /* 最多片数 */
        this.flakeSize = snow.flakeSize || 10;  /* 雪花形状 */
        this.fallSpeed = snow.fallSpeed || 1;   /* 坠落速度 */
  }
   /* 兼容写法 */
   requestAnimationFrame = window.requestAnimationFrame ||
       window.mozRequestAnimationFrame ||
       window.webkitRequestAnimationFrame ||
       window.msRequestAnimationFrame ||
       window.oRequestAnimationFrame ||
       function(callback) { setTimeout(callback, 1000 / 60); };

   cancelAnimationFrame = window.cancelAnimationFrame ||
       window.mozCancelAnimationFrame ||
       window.webkitCancelAnimationFrame ||
       window.msCancelAnimationFrame ||
       window.oCancelAnimationFrame;
   /* 开始下雪 */
   snowFall.prototype.start = function(){
       /* 创建画布 */
       snowCanvas.apply(this);
       /* 创建雪花形状 */
       createFlakes.apply(this);
       /* 画雪 */
       drawSnow.apply(this)
   }
   /* 创建画布 */
   function snowCanvas() {
       /* 添加Dom结点 */
       var snowcanvas = document.createElement("canvas");
       snowcanvas.id = "snowfall";
       snowcanvas.width = window.innerWidth;
       snowcanvas.height = document.body.clientHeight;
       snowcanvas.setAttribute("style", "position:absolute; top: 0; left: 0; z-index: 1; pointer-events: none;");
       document.getElementsByTagName("body")[0].appendChild(snowcanvas);
       this.canvas = snowcanvas;
       this.ctx = snowcanvas.getContext("2d");
       /* 窗口大小改变的处理 */
       window.onresize = function() {
           snowcanvas.width = window.innerWidth;
           /* snowcanvas.height = window.innerHeight */
       }
   }
   /* 雪运动对象 */
   function flakeMove(canvasWidth, canvasHeight, flakeSize, fallSpeed) {
       this.x = Math.floor(Math.random() * canvasWidth);   /* x坐标 */
       this.y = Math.floor(Math.random() * canvasHeight);  /* y坐标 */
       this.size = Math.random() * flakeSize + 2;          /* 形状 */
       this.maxSize = flakeSize;                           /* 最大形状 */
       this.speed = Math.random() * 1 + fallSpeed;         /* 坠落速度 */
       this.fallSpeed = fallSpeed;                         /* 坠落速度 */
       this.velY = this.speed;                             /* Y方向速度 */
       this.velX = 0;                                      /* X方向速度 */
       this.stepSize = Math.random() / 30;                 /* 步长 */
       this.step = 0                                       /* 步数 */
   }
   flakeMove.prototype.update = function() {
       var x = this.x,
           y = this.y;
       /* 左右摆动(余弦) */
       this.velX *= 0.98;
       if (this.velY <= this.speed) {
           this.velY = this.speed
       }
       this.velX += Math.cos(this.step += .05) * this.stepSize;

       this.y += this.velY;
       this.x += this.velX;
       /* 飞出边界的处理 */
       if (this.x >= canvas.width || this.x <= 0 || this.y >= canvas.height || this.y <= 0) {
           this.reset(canvas.width, canvas.height)
       }
   };
   /* 飞出边界-放置最顶端继续坠落 */
   flakeMove.prototype.reset = function(width, height) {
       this.x = Math.floor(Math.random() * width);
       this.y = 0;
       this.size = Math.random() * this.maxSize + 2;
       this.speed = Math.random() * 1 + this.fallSpeed;
       this.velY = this.speed;
       this.velX = 0;
   };
   // 渲染雪花-随机形状（此处可修改雪花颜色！！！）
   flakeMove.prototype.render = function(ctx) {
       var snowFlake = ctx.createRadialGradient(this.x, this.y, 0, this.x, this.y, this.size);
       snowFlake.addColorStop(0, "rgba(255, 255, 255, 0.9)");  /* 此处是雪花颜色，默认是白色 */
       snowFlake.addColorStop(.5, "rgba(255, 255, 255, 0.5)"); /* 若要改为其他颜色，请自行查 */
       snowFlake.addColorStop(1, "rgba(255, 255, 255, 0)");    /* 找16进制的RGB 颜色代码。 */
       ctx.save();
       ctx.fillStyle = snowFlake;
       ctx.beginPath();
       ctx.arc(this.x, this.y, this.size, 0, Math.PI * 2);
       ctx.fill();
       ctx.restore();
  };
  /* 创建雪花-定义形状 */
  function createFlakes() {
      var maxFlake = this.maxFlake,
          flakes = this.flakes = [],
          canvas = this.canvas;
      for (var i = 0; i < maxFlake; i++) {
          flakes.push(new flakeMove(canvas.width, canvas.height, this.flakeSize, this.fallSpeed))
      }
  }
  /* 画雪 */
  function drawSnow() {
      var maxFlake = this.maxFlake,
          flakes = this.flakes;
      ctx = this.ctx, canvas = this.canvas, that = this;
      /* 清空雪花 */
      ctx.clearRect(0, 0, canvas.width, canvas.height);
      for (var e = 0; e < maxFlake; e++) {
          flakes[e].update();
          flakes[e].render(ctx);
      }
      /*  一帧一帧的画 */
      this.loop = requestAnimationFrame(function() {
          drawSnow.apply(that);
      });
  }
  /* 调用及控制方法 */
  var snow = new snowFall({maxFlake:60});
  snow.start();